The  Precipitation  of  the  Sulphides  “of 
Nickel  and  Cobalt  in  an  Alkaline 
T artrate  Solution,  T ogether  with 
an  Investigation  into  the 
Nature  of  Certain  Tar- 
trates of  these 
Metals.  I 


By  O*  F*  Tower. 


"T*  (o  ‘sT’ 


[Reprinted  from  the  Journal  of  the  American  Chemical  Society, 
VoL.  XXII,  No.  8.  August,  1900.] 


THE  PRECIPITATION  OF  THE  SULPHIDES  OF  NICKEL  AND 
COBALT  IN  AN  ALKALINE  TARTRATE  SOLUTION, 
TOGETHER  WITH  AN  INVESTIGATION  INTO 
THE  NATURE  OF  CERTAIN  TARTRATES 
OF  THESE  METALS. 

By  O.  F.  Tower. 

Received  June  19,  1900. 

INTRODUCTION. 

VILLIERS^  recommended  some  time  ago  a method  for  the 
qualitative  separation  of  nickel  and  cobalt  based  on  the 
action  of  hydrogen  sulphide  on  an  alkaline  solution  of  the  tartrates 

1 Compt.  rend.,  84,  946  (1877). 

2 Ibid.,  119,  1263,  and  lao,  46  (1894-95). 


^.'93  7 4 


502 


O.  F.  TOWER. 


of  these  metals.  This  method  as  commonly  carried  out  may  be 
stated  as  follows : The  solution  of  the  sulphides  of  nickel  and 
cobalt  in  aqua  regia  is  evaporated  to  expel  chlorine,  and  after 
suitable  dilution  sufficient  tartaric  acid  is  added  to  prevent  pre- 
cipitation by  sodium  hydroxide.  This  last  reagent  is  then 
added  until  the  solution  is  strongly  alkaline,  and  hydrogen  sul- 
phide run  in  to  saturation.  Cobalt  sulphide  is  precipitated, 
while  nickel  sulphide  remains  in  solution,  imparting  to  the 
solution  a dark  color.  When  nickel  is  present  only  in  very 
small  quantities  the  color  is  brown;  with  larger  quantities  it  is  a 
jet  black. 

The  failure  of  this  method  to  give  good  results  in  the  hands 
of  students,  has  led  to  a critical  investigation  of  it,  the  results  of 
which  are  given  in  the  following  pages.  In  order  to  be  able  to 
approach  this  subject  intelligently  this  account  will  be  preceded 
by  an  account  of  an  investigation  into  the  nature  of  some  tar- 
trate solutions  of  nickel  and  cobalt,  together  with  a description 
of  any  such  tartrates  which  it  has  been  found  possible  to  isolate. 

All  substances  before  being  analyzed  were  dried  in  an  air-bath 
at  120°. 

Nickel  and  cobalt  were  always  determined  electrolytically 
following  in  most  respects  the  method  of  Fresenius  and  Berg- 
niann,^  which  may  be  outlined  as  follows  : To  the  solution  con- 
taining nickel  or  cobalt,  which  is  free  from  chlorides,  are  added 
loo  cc.  ammonia  solution  (sp.  gr.  0.96)  and  10  cc.  of  a solution 
of  ammonium  sulphate  (305  grams  to  liter).  A current  from 
two  storage  battery  cells  (3.2  volts,  0.48  ampere)  was  passed 
through  the  solution  for  from  four  to  five  hours.  This  length 
of  time  was  found  to  be  sufficient  to  effect  complete  precipitation 
of  the  metal  (quantities  not  exceeding  o. 1 100  gram),  provided 
tartrates  were  absent.  It  was  necessary,  however,  to  determine 
these  metals  frequently  in  the  presence  of  tartrates.  In  such 
cases  the  amount  of  metal  deposited  at  the  end  of  four  or  five 
hours  was  weighed,  removed  from  the  electrode,  and  the  appa- 
ratus then  reconnected  and  left  running  all  night.  The  results 
w^ere  then  very  3atisfactory. 

Potassium  was  determined  as  sulphate.  After  removing  the 
nickel  either  by  precipitation  as  sulphide  or  by  electrolysis,  tar- 

1 Ztschr.  anal.  Chem.,v^,  314  (1880). 


SULPHIDES  OF  NICKEL  AND  COBALT.  503 

taric  acid  was  destroyed  by  gentle  ignition,  and  the  potassium 
then  converted  into  sulphate  by  heating  with  ammonium 
sulphate. 

TARTRATES  OF  NICKEL. 

The  effect  of  tartaric  acid  in  preventing  the  precipitation  of 
the  hydroxides  of  nickel  and  cobalt  has  been  known  since  the 
time  of  Rose,^  and  since  then  has  been  frequently  discussed. 
The  literature  on  the  preparation  of  well-defined  tartrates  of 
these  metals  is,  however,  rather  meagre. 

That  nickel  tartrate  cannot  be  precipitated  from  solutions  of 
nickel  salts  is  well  known.  Werther^  prepared  it  by  saturating 
a boiling  solution  of  tartaric  acid  with  freshly  precipitated  nickel 
hydroxide.  The  substance  was  thrown  down  as  a pale  green 
powder  practically  insoluble  in  hot  or  cold  water,  but  soluble  in 
warm  alkalies.  This  has  been  essentially  confirmed.  The  pre- 
cipitate is  apparently  amorphous,  although  Werther  considered 
it  crystalline.  The  filtrate  is  still  colored  green,  showing  that 
precipitation  is  incomplete,  but  what  is  precipitated  is 
extremely  insoluble  in  water.  The  substance  washed  and  dried 
at  120°  yielded  28.44  per  cent,  nickel.  The  theoretical  per- 
centage of  nickel  in  NiC^H^Og  is  28.39.  This  substance  dis- 
solves readily  in  alkalies,  only  when  they  are  present  in  large 
excess.  To  effect  solution,  considerably  more  potassium 
hydroxide  is  required  than  a quantity  equivalent  to  the  nickel 
tartrate  ; that  is,  more  than  two  molecules  potassium  hydroxide 
to  one  nickel  tartrate,  and  the  action  is  greatly  accelerated  by 
heating.  The  behavior  of  this  nickel  tartrate  toward  atmos- 
pheric moisture  is  worthy  of  remark.  The  precipitated  powder 
dried  at  50°  still  contains  moisture.  In  this  condition,  however, 
it  neither  deliquesces  nor  efdoresces,  the  moisture  content 
remaining  constant.  On  the  other  hand  no  definite  hydrate 
seems  to  exist,  for  different  samples  on  drying  at  120°  were 
found  to  contain  different  percentages  of  residual  water.  Fur- 
thermore, the  water-free  substance  is  not  in  the  least  hygro- 
scopic. When  exposed  under  a bell- jar  to  an  atmosphere  satu- 
rated with  moisture,  it  does  not  gain  in  weight  more  than  a 
milligram  or  two  in  several  weeks.  Nickel  tartrate  can  be  made 

1 Gilbert’s  Annalen,  73,  74,  foot-note  (1S23). 

2 J.prakt.  Chem.,  3a,  400  (1844). 


504 


O.  F.  TOWER. 


equally  well  by  treating  a hot  solution  of  tartaric  acid  with 
nickel  carbonate. 

Fresenius^  prepared  a hydrated  racemate  of  nickel  from  nickel 
acetate  and  racemic  acid.  It  resembles  nickel  tartrate  in  some 
of  its  characteristics  but  was  not  further  investigated. 

If  freshly  precipitated  nickel  hydroxide  is  treated  with  cold 
dilute  tartaric  acid,  the  nickel  hydroxide  dissolves,  imparting  to 
the  solution  a green  color,  probably  due  to  the  formation  of  nickel 
tartrate.  On  warming  this  solution  the  light  green  powder 
mentioned  above  precipitates.  To  determine  the  molecular  size 
of  the  substance  in  solution,  a solution  was  prepared  by  treating 
an  excess  of  nickel  hydroxide  with  a known  quantity  of  tartaric 
acid,  and  then  finding  the  freezing-point  of  the  resulting  solution. 
The  reaction  between  nickel  hydroxide  and  tartaric  acid  is  very 
slow,  so  that  after  five  days’  standing  free  tartaric  acid  was  still 
present  in  sufficient  quantity  to  redden  blue  litmus  paper,  2 or 
3 milligrams  of  the  acid  being  uncombined.  This  Was  not 
sufficient  to  influence  materially  the  results  which  follow.  The 
apparatus  employed  for  the  freezing-point  determinations  was 
Beckmann’s  improved  form,^  and  is  particularly  designed  to 
exclude  moisture  from  the  solution  during  the  process.  This 
is  accomplished  by  operating  the  stirrer  by  means  of  an  electro- 
magnet, thereby  obviating  the  necessity  of  having  a hole  through 
the  stopper  for  the  stirring  shaft.  Since  aqueous  solutions  were 
used  in  these  experiments,  the  exclusion  of  moisture  made  very 
little  difference,  but  this  form  of  apparatus  has  other  advantages. 
It  is  exceedingly  convenient,  requiring  the  operator’s  attention 
only  for  a few  minutes  at  the  time  of  freezing,  and  besides  the 
regularity  of  stirring  insures  greater  accuracy.  Individual  deter- 
minations of  the  freezing-point  of  the  same  substance  did  not 
vary  more  than  0.002°.  Five  cells  of  a storage  battery  were 
used  to  supply  power  for  the  electromagnet.  Table  i gives  the 
results  with  solutions  of  nickel  tartrate  prepared  as  indicated 
above.  The  last  four  are  with  different  concentrations  of  the 
same  solution.  The  strength  of  such  solutions  was  always 
determined  by  precipitating  the  nickel  in  an  aliquot  portion  by 
electrolysis. 

1 Ann.  Chem.  (Eiebig),  41,  23  (1842). 

2 Ztschr.  phys.  Chem.,  21,  239  (1896). 


SUI.PHIDKS 

OF  NICKKI.  AND 

cobalt.  505 

Amount  NiC4H406 
in  100  cc.  solution. 
Grams. 

Tabds  I. 

Depression. 

Apparent 
Molecular  weight. 

1.8795 

0.139° 

260 

1.7205 

0.121° 

273 

0.8602 

0.081° 

202 

0.4301 

0.042° 

193 

0.2150 

0.025° 

163 

Molecular 

weight  of  NiQH^Og 

= 206.8. 

In  the  case  of  the  more  concentrated  solutions  these  results 
show  the  molecular  weight  calculated  from  the  lowering  of  the 
freezing-point  to  be  higher  than  that  calculated  from  the  formula. 
One  would  expect,  however,  the  reverse,  because  of  dissociation 
in  aqueous  solution.  By  the  freezing-point  method  Kahlenberg 
has  shown  the  apparent  molecular  weight  of  potassium  tartrate 
in  solutions  of  moderate  strength  to  be  about  one-half  that  cal- 
culated from  the  formula,  These  facts  lead  to  the 

conclusion  that  nickel  tartrate  exists  in  fairly  concentrated  solu- 
tions largely  in  the  form  of  double  molecules. 

Determinations  of  the  electrical  conductivity  of  these  solutions 
show  abnormal  results  also.  In  Table  2,  m is  the  fraction  of  a 
gram-equivalent  Q-NiC^H^Og)  in  a liter;  L is  the  equivalent 
conductivity  expressed  in  reciprocal  ohms.^ 


Temperature  i8°. 

TaBDF  2. 

Temperature  25°. 

Nickel 

tartrate. 

Magnesium  tartrate. 3 

m. 

L. 

m. 

D. 

0.1664 

8.29 

0.03125 

54-9 

0.0832 

9-34 

0.0156 

64.1 

0.0416 

10.8 

0.0078 

74.0 

0.0208 

18.7 

0.0039 

82.6 

0.0104 

26.5 

0.0020 

90.1 

0.0052 

36.8 

0.0010 

95.9 

0.0026 

48.8 

0.0013 

63.6 

The  values  of  the  conductivity  of  nickel  tartrate  are  excep- 
tionally small,  as  will  be  seen  by  comparing  them  with  the 
values  for  magnesium  tartrate,  which  illustrate  normal  conduc- 

1 Ztschr.  phys.  Chem.,  17,  585  (1895). 

2 These  units  are  those  proposed  by  Kohlrausch,  and  fully  described  in  “neitver- 
mogen  der  Klektrolyte,”  by  Kohlrausch  and  Holborn. 

3 Determinations  by  Walden.  Ztschr.  phys.  Chem.,  1,537  (1887),  recalculated  by 
Kohlrausch  and  Holborn. 


5o6 


O.  F.  TOWER. 


tivities  of  tartrates  of  this  class.  The  different  behavior  of 
nickel  tartrate  can  only  be  ascribed  to  a peculiar  constitution  of 
the  salt  itself.  In  seems  reasonable  to  suppose  that  a substance 

COONiOOC 

I I 

CHOH  CHOH 

with  the  atoms  arranged  in  this  way  | | might 

CHOH  CHOH 

I I 

COONiOOC 

well  suffer  levSS  dissociation  than  a normal  tartrate.  Dissociation  in 
concentrated  solutions  would  probably  be  of  the  nature, 

+ + — — 

Ni  and  C^H^OgNiC^H^Og.  On  dilution  this  anion  itself  would 
gradually  be  decomposed,  so  that  the  conductivity  would  not 
advance  with  the  same  regularity  as  it  does  in  the  case  of  simple 
binary  electrolytes.  In  Table  2,  the  results  with  nickel  tartrate 
show  such  behavior,  as  is  seen  by  comparison  with  the  value  for 
magnesium  tartrate.  Such  a formula  as  the  above  will  also 
explain  very  satisfactorily  the  results  obtained  from  the  freezing- 
point  method.  Measurements  of  the  electromotive  forces  with 
solutions  of  this  kind  will  not  be  given,  until  the  facts  relating 
to  some  other  tartrates  of  nickel  have  been  discussed. 

Fabian^  prepared  potassium  nickel  tartrate  by  allowing  cream 
of  tartar  to  act  on  nickel  carbonate  at  a temperature  of  about  50°. 
On  evaporating  the  solution  obtained  over  sulphuric  acid  a 
greenish  substance  was  deposited,  which  effloresced  on  exposure 
to  air  and  was  soluble  in  water.  After  drying  this  substance  at 
110°  and  analyzing,  he  obtained  the  following  results  ; 

Found.  Theory  for  NiK2C8H80i2. 

Per  cent.  Per  cent. 


NiO 16.8  17.3 

K2O 20,9  21.7 


This  work  has  been  repeated  with  essentially  the  same  results. 
Cream  of  tartar  was  allowed  to  stand  in  contact  with  an  excess 
of  nickel  carbonate  at  about  40°  for  some  time.  The  green  solu- 
tion was  filtered  off,  and  left  to  evaporate  over  sulphuric  acid. 
No  definite  crystals  were  obtained,  neither  by  evaporating 
rapidly  in  a partial  vacuum  nor  slowly  in  the  air  with  a string. 
The  residue  was  invariably  of  a scaly  appearance  possessing 

1 Ann.  Chem.  (Liebig),  103,  248  (1857). 


SULPHIDES  OE  NICKEL  AND  COBALT.  507 

a very  light  green  color.  The  results  of  the  analysis  of  this 
substance  are  : 

Found.  Theory  for  KjNiCgHgOij. 

Per  cent.  Per  cent. 


K 18.21  18.06 

Ni 13.38  13.55 


According  to  Fabian,  on  boiling  a solution  of  this  substance 
a gelatinous  mass  separates  out,  which  cannot  be  washed  com- 
pletely free  from  alkali.  This  has  been  found  to  be  true,  the 
precipitate  being  rather  flocculent  and  of  a very  light  green 
color.  If,  however,  a solution  of  potassium  nickel  tartrate  is 
digested  for  some  time  at  about  75°,  a light  green  pulverulent 
precipitate  can  be  obtained,  which  was  supposed  to  be  a basic 
tartrate  of  nickel.  After  washing  it  thoroughly  and  drying  at 
120°,  analysis  proved  it  to  be  identical  with  'the  insoluble  nickel 
tartrate  already  described,  a sample  yielding  28.25  cent, 
nickel.  On  long  standing  this  decomposition  takes  place  grad- 
ually at  lower  temperatures.  It  is,  therefore,  necessary  to  exer- 
cise care  in  the  preparation  of  potassium  nickel  tartrate,  for,  if 
tartaric ‘acid  is  digested  for  a long  time  with  nickel  carbonate, 
some  of  the  potassium  nickel  tartrate  formed  is  apt  to  decompose 
in  this  manner : 

Insoluble  form 

K,mc,u,0,,  = NiC^HA  A-  K,C,H,Oe- 

When  the  solution  is  filtered,  the  potassium  tartrate  passes 
through  with  the  potassium  nickel  tartrate,  and  remains  mixed 
with  it  on  evaporation.  This  is  revealed  by  the  percentage  of 
potassium  being  too  high  and  that  of  nickel  too  low  to  correspond 
to  the  formula  K,^NiC8HgOj2. 

Determinations  of  the  molecular  weight  of  this  substance  were 
made  by  means  of  the  freezing-point  method.  For  this  purpose 
different  solutions  were  employed, — some  prepared  as  above 
described,  some  by  dissolving  the  solid  substance  in  water,  and 
others  by  dissolving  nickel  hydroxide  in  the  proper  amount  of 
tartaric  acid  and  adding  that  quantity  of  potassium  hydroxide 
just  sufficient  to  form  K2NiCgHgOi2.  The  results  follow  : 


5o8 


O.  F.  TOWER. 


TabIvK  3. 


NiCOg  and  HKQH4O6 

Solid  substance  dissol 
water 


Amount 
K^NiCgHgOi^ 
in  100  cc. 
solution. 

Depres- 

Apparent 

molecular 

Grams. 

sion. 

weight. 

3tn  ' 

r 3-764 

0.434° 

162 

1.882 

0.255 

140 

. . . 1 

1 0.941 

0.170 

105 

in  i 

r 3-752 

0.430 

165 

. . 1 

L 1.876 

0.253 

140 

r 3-295 

0.419 

153 

1 1-6475 

0.246 

128 

' 0.8238 

0.139 

II2 

I0.4II9 

0.079 

98 

Molecular  weight  of  K2NiCgHgOi2  = 433- 


It  is  essential  to  know  something  about  the  dissociation  of 
this  substance  before  one  can  judge  intelligently  of  its  true 
molecular  size.  To  throw  light  on  this  subject  the  electrical 
conductivity  of  a solution  of  potassium  nickel  tartrate  was 
measured.  The  equivalent  conductivity  is  given  on  the  basis  of 
one  potassium  atom  (|  molecule  K2NiCgHg042  in  a liter). 


Tabi,e  4. 

Temperature  18°.  Temperature  25“. 

Potassium  nickel  tartrate.  Sodium  tartrate. 1 


m. 

Iv. 

m. 

D. 

0.1522 

74-6 

0.0761 

84.3 

0.0381 

94.2 

0.03125 

87.1 

0.0190 

lOI 

0.0156 

93-2 

0.0095 

114 

0.0078 

98.2 

0.0048 

124 

0.0039 

102. 1 

The  conductivity  of  the  double  tartrate  is  increasing  with  the 
dilution  much  faster  than  that  of  the  sodium  tartrate.  This  is 
undoubtedly  due  to  the  dissociation  of  potassium  nickel  tartrate 

mostly  into  K ions  and  NiCgHgOi2  ions  in  the  more  concentrated 
solutions.  On  dilution,  however,  the  nickel  also  begins  to  dis- 
sociate from  the  complex  anion,  thus  causing  the  rapid  increase 
in  the  conductivity.  For  corresponding  concentrations  the  con- 
ductivity of  the  double  tartrate  always  exceeds  that  of  the 
sodium  tartrate. 

Kahlenberg,^  in  his  article  on  complex  tartrates  of  lead  and 

1 The  conductivity  of  sodium  tartrate  is  given  for  comparison.  The  results  are  those 
of  Bredig,  Ztschr.  phys.  Chem.,  13,  igi  (1894),  recalculated  by  Kohlrausch  and  Holborn. 

2 Ztschr.  phys.  Chem.,  1 7.  577  (1895). 


SUI.PHIDES  OF  NICKEE  AND  COBAET.  509 

copper,  found  that  these  substances  were  probably  dissociated 
sufficiently,  so  that  the  apparent  molecular  weight  was  about 
one  half  the  true  molecular  weight.  The  solutions  of  potassium 
nickel  tartrate  used  here  were,  however,  a little  more  dilute  than 
Kahlenberg’s  solutions  ; besides  the  substances  are  somewhat 
different  in  nature.  Table  3 shows  the  apparent  molecular 
weight  to  be  only  a little  more  than  ^ of  the  molecular 
weight  of  K2NiCgHg0^2.  This  may  be  accounted  for  on  two 

hypotheses  ; either  the  substance  is  in  large  part  dissociated  into 

+ + 

K,  K and  NiCgHgO^^  as  was  indicated  in  connection  with 
the  conductivity  measurements,  or  there  exist  separately  in  the 
solution  potassium  tartrate  and  nickel  tartrate,  the  former  dis- 
sociated, the  latter,  however,  not.  The  evidence  seems  to  favor 
the  former  of  these  hypotheses.  For,  if  the  second  were  correct, 
one  would  expect  the  deposit  left  on  evaporation  of  the  solution 
not  to  have  a uniform  composition.  But  the  deposit  on  the  side 
of  the  dish  was  of  the  same  composition  as  that  on  the  bottom. 
Furthermore,  washing  the  deposit  slightly  with  water  had  no 
effect  on  its  composition.  If  the  substance  had  been  a mixture 
of  potassium  tartrate  and  nickel  tartrate,  this  treatment  would 
very  likely  dissolve  more  of  the  former  salt  than  of  the  latter. 

The  fact  that  the  addition  of  tartaric  acid  to  solutions  of 
nickel  salts  prevents  the  precipitation  of  nickel  hydroxide  is 
usually  explained  by  the  supposition  that  in  such  cases  the 
nickel  replaces  the  hydrogen  atoms  of  the  alcoholic  hydroxyls 
of  the  tartaric  acid,  while  the  potassium  takes  the  place  of  the  two 
carboxyl  hydrogen  atoms.  The  formula  of  such  a substance 
COOK 

CHO 

would  be  I >Ni.  To  see  if  any  experimental  basis  can  be 
CHO 
I 

COOK 

found  for  this  view,  solutions  of  nickel  tartrate,  to  which  potas- 
sium hydroxide  had  been  added,  were  investigated  by  the  usual 
physico-chemical  methods. 

The  freezing-point  method  was  first  employed  in  order  to 
observe  the  effect  on  the  size  of  the  molecule  of  successive 


O.  F.  TOWER. 


510 

additions  of  potassium  hydroxide^  to  a solution  of  nickel  tartrate. 

Table  5. 


Solution  containing 

1.8795  grams  nickel 

tartrate  in  100  cc. 

Expt. 

No. 

KOH  added  to 
above  solution. 
Gram. 

Depression. 

Remarks. 

I 

0.0000 

0.139° 

Reaction  acid. 

2 

0.1226 

0.180 

Reaction  alkaline. 

3 

0.3270 

0.187 

4 

0.5314 

0.134 

5 

0.7358 

0.180 

Solution  jelly-like. 

6 

, 0.8175 

0.156 

7 

I.3II 

0.233 

In  Experiment  7,  the  quantity  of  potassium  hydroxide  is  just 
sufficient  to  form,  with  the  nickel  tartrate,  K2NiC^H20g.  In  the 
fourth  the  depression  is  a minimum,  and  here  the  amount  of 
potassium  hydroxide  added  is  nearly  one  half  that  added  in  the 
seventh,  or  the  ratio  is  about  one  molecule  potassium  hydroxide 
to  one  nickel  tartrate.  If  the  molecular  weight  is  calculated 
from  the  depression,  0.134°,  on  the  basis  of  NiKC^HgOg  existing 
in  solution,  299  is  obtained.  The  molecular  weight  according 
to  the  above  formula  is  in  round  numbers  245.  This  case  is 
almost  exactly  parallel  to.  the  results  obtained  by  Kahlenberg^ 
with  certain  alkaline  tartrate  solutions  of  lead  and  copper.  So 
undoubtedly  the  molecular  size  of  the  substance  here  is  about 
twice  that  represented  by  the  formula  NiKC^HgOg,  and  following 
Kahlenberg  the  molecular  structure  may  be  represented  as 


COOK 

KOOC 

I 

CHOH 

j 

HOHC 

follows  : 1 

1 . 

Such  a substance  would 

CHOH 

HOHC 

I 

COO— Ni- 

j 

-0— Ni— OOC 

* 

hardly  be  expected  to  dissociate  so  as  to  yield  nickel  ions.  The 
evidence  for  such  a molecular  structure  would,  therefore,  be 
increased,  if  it  could  be  shown  that  no  nickel  ions  exist  in  the 
solution. 

For  this  purpose  measurements  were  made  of  the  electro- 
motive force  generated  by  an  element  with  nickel  electrodes,  one 

1 Sodium  hydroxide  was  also  employed . but  since  the  results  are  similar,  only  those 
with  potassium  hydroxide  are  given. 

2 Loc  cit. 


SULPHIDES  OF  NICKEL  AND  COBALT. 


511 


electrode  being  surrounded  by  a solution  of  a nickel  salt  of 
known  concentration  and  the  other  surrounded  by  the  solution 
in  question.  The  electromotive  force  of  such  an  element  is 
expressed  by  the  formula, 


RT 


In 


where  the  letters  indicate  the  usual  quantities,  q and  q being  the 
concentration  of  the  nickel  ions  in  the  two  solutions.  Eval- 
uating the  equation  for  R = 8.311  electrical  units,  T = 291  (18° 
C.),  n,  the  valence  of  nickel  — 2,  'Eq  — 96,540  coulombs,  and 
reducing  to  Briggs’  system  of  logarithms,  one  obtains, 

N C 

7t  — 0.02881  log  ~ . 


Measurements  were  made  according  to  the  Poggendorf-Ostwald 
method  with  a Tippmann  capillary  electrometer.  The  nickel 
solution  of  constant  concentration  was  in  every  case  a solution 
of  nickel  nitrate  containing  o.  i gram-molecule  in  a liter.  The 
electrodes  were  made  by  covering  platinum  electrodes  with 
nickel  by  electrolysis.  These  electrodes  were  tested  by  connect- 
ing them  with  an  electrometer,  when  they  were  dipping  into  the 
same  solution  of  nickel  nitrate.  Under  these  conditions  the 
electrometer  reading  should  be  zero.  Few  electrodes,  however, 
could  be  found  which  would  fulfil  this  condition.  Those  chosen 
showed  potential  differences  of  not  more  than  0.003  volt.  Meas- 
urements w’ere  made  first  of  the  electromotive  forces  between 
the  standard  solution  and  solutions  of  nickel  nitrate  containing 
0.05  and  o.oi  gram-molecule  in  a liter,  respectively,  and  after- 
wards with  the  solutions  of  nickel  tartrate  under  consideration. 
In  each  case  the  electrodes  were  reversed  and  the  potential  differ- 
ence measured  again.  The  averages  of  the  two  readings  so 
obtained  are  given  in  Table  6. 


Table  6. 


Temperature  18^. 


7T  TT 


No. 

Between  o.i  mol.  Ni(NOs)2  and 

observed. 

Volt. 

calculated. 

Volt. 

I 

0.05 

mol.  Ni(N03)2 

0.010 

0.0087 

2 

O.OI 

0.032 

0.0288 

3 

0.0208 

“ (NiC,H,Oe)2 

0.048 

4 

0.0761 

“ NiK^CgHgOi^ 

0.075 

5 

0.0572 

“ KQH,06Ni0Ni0eH4C4K2 

0.210 

6 

0.0748 

“ K2NiC4H20e 

0.236 

512 


O.  F.  TOWER. 


Where  the  concentration  of  the  nickel  ions  was  known  the 
electromotive  force  has  been  calculated  from  the  formula.  The 
agreement  is  seen  to  be  fairly  good.  Nos.  3 and  4 are  measure- 
ments with  the  solutions  already  mentioned  of  nickel  tartrate  and 
potassium  nickel  tartrate,  respectively.  The  potential  differences 
show  that  the  concentration  of  the  nickel  ions  in  these  solutions 
is  somewhat  less  than  in  a o.oi  mol.  solution  of  nickel  nitrate. 
Fewer  nickel  ions  are  present  in  the  potassium  nickel  tartrate 
solution  than  in  the  solution  of  nickel  tartrate,  which  was  to  be 
expected,  for  the  same  reason  that  the  replaceable  hydrogen  atom 
of  an  acid  salt  is  only  very  slightly  dissociated.^ 

To  find  the  concentration  of  the  nickel  ions  in  solution  5,  which 
is  the  one  formed  by  adding  to  100  cc.  of  a solution  containing  18.795 
grams  of  nickel  tartrate,  0.5314  gram  of  potassium  hydroxide,'* 
0.210  is  substituted  for  n in  the  formula  and  is  made  equal  to 
o.i  X 0.8,  foro.i  mol.  nickel  nitrate  is  about  80  per  cent,  dis- 
sociated. From  this  the  value  of  c.^  is  found  to  be  10  386.  Xhis 

shows  that,  relatively  speaking,  no  nickel  ions  are  in  the  solu- 
tion, which  is  corroborative  evidence  of  the  existence  of  the 
molecular  structure  ascribed  to  this  substance  on  page  510. 

Referring  to  Table  5,  it  will  be  noticed  that  the  depression 
gradually  increases  with  the  addition  of  potassium  hydroxide 
from  the  fourth  experiment  to  the  seventh,  except  in  No.  5.  In 
this  case  the  contents  of  the  tube  were  viscous  having  the  con- 
sistency of  thin  jelly,  which  interfered  materially  with  the 
determination.  This  gelatinous  mass  completely  dissolved  on 
addition  of  more  potassium  hydroxide.  The  appearance  of  this 
substance  is  peculiar.  The  green  liquid  simply  solidifies  to  a 
transparent  mass  without  any  change  in  appearance.  On 
reversing  the  tube  containing  the  substance  it  is  found  to  be 
solid  or  viscous  according  to  the  amount  of  water  present, 
and  this  is  the  only  evidence  of  a change.  On  standing  for  a 
length  of  time  it  gradually  becomes  opaque.  Several  years  ago 
this  phenomenon  was  observed  by  Professor  Bradley,  of  Middle- 
town,  who  called  my  attention  to  it  at  that  time.  He  obtained 
it  by  adding  first  tartaric  acid  to  a solution  of  nickel  sulphate 
and  then  adding  a solution  of  potassium  hydroxide  drop  by  drop 

1 For  the  discussion  of  this,  see  Ostwald  : Ztschr.  phys.  Chem.,  9,  553  (1892). 

2 See  Table  5. 


SULPHIDES  OF  NICKEL  AND  COBALT. 


513 


until  the  mass  gelatinized.  As  can  be  seen  from  Table  5,  it  is 
formed  when  about  one  molecule  of  nickel  tartrate  is  treated 
with  one  molecule  of  potassium  hydroxide.  It  was  shown 
just  before  the  solution  gelatinized  (experiment  4,  Table  5), 
that  from  the  depression  of  the  freezing-point  and  other  consider- 
ations the  molecule  probably  is  KC^H^OgNiONiOgH^C^K.  It  is, 
therefore,  possible  that  this  gelatinous  mass  is  a hydrated  form 
of  that  substance.  Heating  almost  to  the  boiling-point  com- 
pletely dissolves  it.^ 

To  return  to  the  effect  on  the  depression  of  the  freezing-point 
of  adding  potassium  hydroxide,  as  seen  in  Table  5 from  Experi- 
ment 4 on.  Leaving  out  of  consideration  No.  5,  where  the 
solution  was  viscous,  the  increase  in  the  depression  from  0.134° 
to  0.156°  and  in  the  last  case  to  0.233°  is  by  no  means  sufficient 
to  be  equal  to  the  entire  effect  of  the  potassium  hydroxide 
added,  if  it  existed  as  such  in  the  solution.  0.4935  gram  more 
of  potassium  hydroxide  was  added  in  Experiment  No.  7 than  in 
No.  6.  This  quantity  of  potassium  hydroxide  should  give 
normally  a depression  of  0.121°,^  but  the  difference  between 
0.233°  and  o.  156°  is  only  0.077°.  7 proportion  of 

potassium  hydroxide  to  nickel  tartrate  is  two  molecules  of  the 
former  to  one  of  the  latter,  or  in  the  proper  ratio  to  form 
KOOC 

I 

OCH 

Ni<  I , the  molecular  weight  of  which  is  283.  The  apparent 
OCH 


KOOC 

^ since  writing  this,  Prof.  Bradley  has  kindly  allowed  me  to  look  over  an  unpub- 
lished manuscript  by  himself  and  F.  A.  Johnston  on  “A  Tartrate  of  Sodium  and 
Nickel,”  which  treats  of  the  formation,  conduct,  and  gives  some  results  of  the  analysis 
of  this  gelatinous  substance.  It  was  prepared  from  nickel  sulphate,  tartaric  acid,  and 
sodium  hydroxide.  It  was  isolated  by  stirring  it  up  in  alcohol,  filtering  and  then  wash- 
ing with  an  equal  volume  of  alcohol  and  water.  When  so  treated  the  gelatine  frequently 
remains  transparent  for  a long  time.  It  was  found  difficult  to  dry  it  to  constant  weight 
over  sulphuric  acid,  for  it  is  exceedingly  hygroscopic.  Preliminary  analysis  of  the  sub- 
stance after  drying  over  sulphuric  acid  showed  : water,  7.04  per  cent.;  sodium,  6.30  per 
cent.;  nickel,  21.04  per  cent.  We  obtain  by  calculation  from  the  above  formula  plus  2 
molecules  of  water,  NaaNi-^CgHgOja  -|-  2HsO  : water,  7.04  per  cent.;  sodium,  9.00  per 
cent.;  nickel,  23.95  per  cent.  The  most  marked  difference  between  these  results  and  the 
results  of  the  analysis  is  in  the  percentage  of  sodium.  This  may  be  accounted  for  from 
the  fact  that  the  gelatine  is  strongly  alkaline  and  that  washing  removes  some  of  this 
alkali,  so  that  the  residue  may  not  have  a constant  composition.  However,  it  is  not 
asserted  that  the  above  formula  represents  the  composition  of  the  substance,  for  more 
analyses  are  needed  in  which  the  amount  of  tartaric  acid  is  also  determined,  before  the 
differences  in  the  above  results  are  explained  and  a suitable  formula  established. 

2 Calculated  from  the  constant  for  potassium  hydroxide  reported  by  Raoult,  Ann. 
chim.phys.  (5),  28,  137  (1883). 


514 


O.  F.  TOWER. 


molecular  weight  calculated  from  the  depression,  0.233  is  185, 
which  is  a little  more  than  half  that  calculated  from  the  formula. 
Since  such  a substance  would  probably  be  dissociated  in  aqueous 
solution  about  as  much  as  potassium  tartrate,^  it  is  very  likely 
that  a substance  of  the  above  formula  does  really  exist  in  the 
solution.  Nickel  ions  would  not  be  expected  to  be  present  in 
any  quantity  in  a solution  of  such  a substance.  The  potential 
difference  found  between  this  solution  and  o.  i mol.  nickel 
nitrate  is  0.256  volt  (No.  6,  Table  6).  This  shows  that  the  con- 
centration of  the  nickel  ions  is  even  less  than  in  the  preceding 
solution  (No.  5),  where  the  concentration  of  these  ions  was 
found  to  be  What  has,  therefore,  probably  taken  place 

on  the  addition  of  potassium  hydroxide  from  solution  4 to  solu- 
tion 7,  Table  5,  is  that,  although  the  potassium  hydroxide  is 
continually  being  removed  from  the  field  of  action  as  such,  the 
molecular  structure  of  the  substance  in  solution  is  undergoing  a 
change;  that  is,  the  complex  molecule,  KC^H^OgNiONiOgH^C^K, 
is  breaking  up  into  the  smaller  molecule,  K2NiC4HgOg ; the 
number  of  molecules  is,  consequently,  on  the  whole  increased, 
and  the  depression  of  the  freezing-point  becomes  greater. 

TARTRATES  OF  COBAET. 

Very  little  literature  of  a definite  nature  could  be  found  on 
this  subject.  A tartrate  of  cobalt  and  a double  tartrate  of 
potassium  and  cobalt  are  mentioned  in  the  following  dictionaries 
of  chemistry:  Fehling’s,  Vol.  9 (1864)  ; Watts’,  Vol.  5 (1868)  ; 
Wurtz’s,  Vol.  3 (1878).  Nearl}^  all  the  early  journals  and  such 
periodicals  as  Berzelius’  Jahresbericht,  Liebig  and  Kopp"^s  Jahres- 
bericht^  the  Pharmaceutisches  Centralblatt^  &c.,  were  thoroughly 
searched,  but  no  reference  was  found  to  the  original  articles 
describing  these  salts.  Such  w^orks  as  Ladenburg’s  Dictionary, 
edition  1894,  and  Beilstein’s,  third  edition,  mention  no  such 
substances.  It  is,  therefore,  to  be  inferred  that  the  original 
article  contained  nothing  very  definite.  Fresenius^  prepared 
certain  racemates  of  cobalt,  but  gives  no  analyses. 

Cobalt  tartrate  can  be  made  in  the  same  manner  as  nickel 
tartrate,  by  saturating  hot  tartaric  acid  with  cobalt  hydroxide 

1 See  Kahlenberg,  loc.  cit. 

2 A7in.  Chem.  (Liebig),  41,  22  (1842). 


SULPHIDES  OF  NICKEL  AND  COBALT. 


515 


or  carbonate.  It  is  thown  down  as  a pale  pink  powder  prac- 
tically insoluble  in  hot  or  cold  water,  and  possessing  in  general 
the  other  properties  of  nickel  tartrate.  A sample  of  this  dried 
at  120°  gaye  28.32  per  cent,  cobalt  (theory,  28.49  cent.). 

When  cobalt  hydroxide  is  treated  in  the  cold  with  tartaric 
acid  it  dissolves  to  a certain  extent  a pink  solution  resulting, 
which,  on  standing,  deposits  reddish  pink  crystalline  scales  of 
cobalt  tartrate.  This  crystalline  substance  differs  from  the 
cobalt  tartrate  just  described  by  being  somewhat  soluble  in 
water.  It  is  impossible  to  obtain  as  concentrated  solutions  of 
cobalt  tartrate  as  of  nickel  tartrate,  for  the  reason  just  mentioned, 
that  some  of  the  salt  crystallizes  out.  With,  however,  as  strong 
a solution  as  could  be  obtained  the  following  determinations 
were  made  of  the  depression  of  the  freezing-point. 


Amount  CoC4H40e 
in  100  cc.  solution. 
Gram. 

0.6505 

0.3252 

0.1626 


Table  7. 

Depression. 

0.059° 

0.036 

0.021 


Apparent 
molecular  weight. 

207 

170 

146 


Molecular  weight  of  CoQH^Og  = 207.1. 


These  results  run  about  the  same  as  those  obtained  with  solu- 
tions of  nickel  tartrate  of  similar  concentration.  The  apparent 
molecular  weight,  207,  is  very  nearly  equal  to  the  molecular 
weight  calculated  from  the  formula;  therefore,  for  the  reasons 
already  mentioned,  the  true  molecular  weight  of  cobalt  tartrate 
is  probably  twice  207.1,  which  means  it?  structure  would  be 
represented  by  twice  the  above  formula  or  (CoC4H406)2.  Table 
8 gives  the  results  of  the  determination  of  the  electrical  con- 
ductivity of  solutions  of  this  substance.  The  units  are  on  the 
basis  of  equivalent  (4-  CoC^H^O^;)  in  a liter  as  before. 


Table  8. 


Temperature  i8'. 


m. 

L. 

0.0628 

15-9 

0.0314 

22.1 

0.0157 

28,7 

0.0079 

37-6 

0.0039 

47.1 

0.0020 

58.4 

0.0010 

69.9 

0.0005 

82.1 

O.  F.  TOWER. 


516 

These  results  are  entirely  analogous  to  the  conductivity  of 
nickel  tartrate  (Table  2).  They  are  abnormally  small  and  show 
that  cobalt  tartrate  is  only  slightly  dissociated,  which  affords 
additional  evidence  that  the  molecule  is  polymerized. 

Cream  tartar  acts  vigorously  on  cobalt  carbonate  in  the 
presence  of  water,  the  resulting  solution  being  colored  a deep 
pink.  When  this  solution  is  evaporated  over  sulphuric  acid, 
reddish  pink  crystalline  scales  are  deposited,  very  similar  in 
appearance  to  the  deposit  from  a solution  of  cobalt  tartrate. 
When  almost  to  dryness  the  solution  assumes  a pasty  nature, 
and  can  be  completely  dried  only  with  considerable  difficulty. 
No  salt  of  the  formula,  K2CoCgHgOi2,  could  be  isolated.  The 
crystalline  deposit  just  mentioned  yielded  on  drying  24.53 
cent,  cobalt,^  and  a small  quantity  of  potassium  was  also  found. 
The  deposit,  therefore,  consists  largely  of  cobalt  tartrate,  but  a 
small  amount  of  potassium  tartrate  is  mixed  with  it.  Watts’ 
“Dictionary,”  and  the  others  above  cited,  say  that  potassio- 
cobaltous  tartrate  forms  large  rhomboidal  prisms.  No  such 
crystals  could  be  obtained  by  any  of  the  methods  used  here. 
Determinations  of  the  freezing-point  of  solutions  prepared  from 
cream  tartar  and  cobalt  carbonate  yielded  essentially  the  same 
results  as  similar  solutions  prepared  from  nickel  carbonate. 
Such  results  might  well  be  in  harmony  with  the  separate 
existence  in  the  solution  of  potassium  tartrate  and  cobalt  tar- 
trate, so  no  definite  knowledge  of  the  nature  of  the  molecules  in 
solution  could  be  obtained  by  this  method.  Heating  this  solu- 
tion causes  the  precipitation  of.  a light  pink  powder.  After 
washing  this  and  drying  at  120°,  28.38  per  cent,  cobalt  was 
found  which  showed  it  to  be  cobalt  tartrate.  This  substance 
possesses  all  the  properties  and  is  apparently  identical  with  the 
cobalt  tartrate  previously  mentioned.^  It  is  to  be  noted  that  the 
conduct  of  this  solution  on  heating  is  somewhat  different  from 
that  prepared  from  nickel  carbonate  and  cream  tartar,  for  a 
flocculent  precipitate  resulted  on  heating  this  latter  to  boiling. 

When  potassium  hydroxide  is  added  to  a solution  of  cobalt 
tartrate  the  conduct  is  very  similar  to  that  when  it  is  added  to  a 
solution  of  nickel  tartrate.  It  differs,  however,  in  this  respect, 

1 The  theoretical  percentage  of  cobalt  according  to  formula  KaCoCgHgOia,  is  13.61. 

2 That  prepared  by  treating  hot  tartaric  acid  with  cobalt  hydroxide. 


SULPHIDES  OE  NICKEL  AND  COBALT.  517 

that  when  the  quantities  of  potassium  hydroxide  and  cobalt  tar- 
trate are  in  the  proportion  of  one  molecule  of  the  former  to  one 
of  the  latter,  instead  of  obtaining  a gelatinous  mass,  a flocculent 
precipitate  results,  which  readily  dissolves  on  further  addition  of 
potassium  hydroxide.  The  range  of  the  existence  of  this  pre- 
cipitate is  considerable  ; that  is,  it  begins  to  form  when  the 
amounts  of  the  two  substances  are  in  about  the  ratio  of  one-half 
molecule  of  potassium  hydroxide  to  one  of  cobalt  tartrate,  and 
does  not  entirely  disappear  until  the  potassium  hydroxide  is 
present  in  about  the  ratio  of  molecules  to  i of  the  cobalt  tar- 
trate. This  seriously  interfered  with  following  the  effect  on  the 
freezing-point  of  successive  additions  of  potassium  hydroxide  to 
cobalt  tartrate.  There  is  no  difficulty,  however,  in  determining 
the  freezing-point  of  the  solution  when  the  potassium  hydroxide 
is  present  in  the  ratio  of  two  molecules  of  it  to  one  of  the  tar- 
trate ; the  results  thus  obtained  are  given  in  Table  9. 


TABrK  9. 

Amount  C0C4H4OB 

Amount 

in  100  cc.  solution. 

KOH  added. 

Gram. 

Gram. 

Depression. 

0.6505 

0.0000 

0.061° 

0.6505 

0.4530 

0.106 

If  the  molecular  weight  is  calculated  from  the  depression 
0.106°,  it  gives  159,  while  according  to  the  formula,  K2CoC4H20g, 
the  molecular  weight  is  283.3.  Owing  to  the  dissociation  which 
such  a salt  would  suffer  in  aqueous  solution,  it  is  very  probable 
that  it  exists  here  in  the  solution,  and  that  it  is  due  to  its  forma- 
tion that  cobalt  hydroxide  cannot  be  precipitated  by  alkalies 
from  a solution  of  a cobalt  salt  to  which  tartaric  acid  has  been 
added.  The  precipitate  above  referred  to,  formed  when  the 
amount  of  potassium  hydroxide  added  to  the  cobalt  tartrate  is 
in  the  ratio  of  one  molecule  of  the  former  to  one  of  the  latter, 
was  filtered  off  and  washed  until  the  wash-water  was  no  longer 
alkaline.  It  was  then  dried  and  the  cobalt  in  it  determined.  In 
one  sample  the  percentage  of  cobalt  was  42.78  and  in  another 
38.04.  The  precipitate  has,  therefore,  no  definite  composition, 
but  undoubtedly  contains  some  cobalt  hydroxide,  for  no  con- 
ceivable tartrate  of  cobalt  has  as  great  a percentage  of  cobalt  as 
was  found. 

5-22 


O.  F.  TOWKR. 


518 


SOIvUBFE  NICKEE  SUEPHIDE. 

It  is  evident  that  when  investigating  the  effect  of  an  alkaline 
tartrate  solution  in  preventing  the  precipitation  of  nickel  sul- 
phide by  hydrogen  sulphide,  it  is  desirable  to  exclude  the  pres- 
ence of  other  chemical  substances.  Tartaric  acid  and  sodium 
hydroxide  were,  consequently,  not  added  to  a solution  of  a 
nickel  salt,  as  for  instance  the  chloride,  for  this  would  produce 
some  sodium  chloride,  whose  influence  on  the  precipitation  with 
hydrogen  sulphide  w^ould  then  be  entirely  unknown.  Solutions 
of  pure  nickel  tartrate  were  prepared  as  has  already  been  indi- 
cated, and  to  this  the  alkali  and  hydrogen  sulphide  were  then 
added.  If  it  was  desired  to  note  the  effect  of  any  neutral  salt, 
as  sodium  chloride,  on  the  precipitation,  such  salt  was  added 
separately.  All  the  nickel  tartrate  used  was  made  from  nickel 
nitrate,  which  was  a preparation  of  Schuchardt’s  and  was 
marked  ‘ 'purissimum.  ’ ’ According  to  Villiers,^  sodium  hydroxide 
is  more  effective  in  holding  nickel  sulphide  in  solution  than 
potassium  hydroxide.  Solutions  of  nickel  tartrate  were  treated 
with  one  equivalent  of  sodium  hydroxide,^  with  two  equivalents 
of  sodium  hydroxide,  with  three  equivalents,  and  so  on. 
Hydrogen  sulphide  was  run  into  these  solutions  to  saturation 
and  the  solutions  filtered.  In  the  first  solution  nickel  sulphide 
was  precipitated  and  the  filtrate  was  only  slightly  colored,  in  the 
second  the  filtrate  was  a decided  brown,  in  the  third  it  was  black, 
but  to  be  certain  that  no  nickel  sulphide  is  precipitated  it  is 
necessary  to  have  at  least  five  equivalents  of  sodium  hydroxide 
to  one  of  nickel  tartrate.  This  black  filtrate  oxidizes  with 
remarkable  rapidity,  which  is  shown  by  the  continual  separation 
of  sulphur.  It  is  apparently  the  sodium  sulphide  present, 
which  thus  suffers  oxidation,  but  what  is  peculiar,  is  that  the 
oxidation  is  more  rapid  than  ever  takes  place  in  a solution  of 
sodium  sulphide  alone.  The  more  dilute  the  solution,  the  less 
marked  is  the  oxidation.  If  this  black  alkaline  solution  of 
nickel  sulphide  is  allowed  to  stand  for  an  indefinite  length  of 
time,  the  nickel  sulphide  gradually  precipitates,  so  that  in  from 
five  to  ten  days  the  supernatant  liquid  has  usually  become  clear. 

1 Compt.  rend.,  119,  1263  (1894). 

2 By  an  equivalent  of  sodium  hydroxide  is  meant  that  quantity  which  bears  a ratio 
to  the  nickel  tartrate  used  of  2 molecules  NaOH  to  i molecule  NiC4H40e. 


SUIvPHIDES  OF  NICKEE  AND  COBAET.  519 

This  settling  can  be  greatly  accelerated  by  the  presence  of  a 
neutral  salt,  as  sodium  chloride  or  potassium  sulphate,  the  time 
necessary  for  settling  varying  with  the  amount  of  such  salt 
added.  The  presence  of  a large  quantity  of  these  salts  before 
the  introduction  of  hydrogen  sulphide  causes  a large  portion  of 
the  nickel  sulphide  to  be  precipitated. 

The  behavior  of  this  black  solution  of  nickel  sulphide  seemed 
to  indicate  that  it  is  not  a true  solution,  but  colloidal  in  nature. 
To  decide  this  several  tests  were  applied.  With  a high-power 
microscope  it  was  impossible  to  detect  solid  particles  of  any  sort. 
The  following  test  employed  by  Muthmann^  was  applied  : It  con- 
sists in  treating  the  solution  in  question  with  a solution  of  gum- 
arabic,  shaking  well,  and  then  adding  alcohol  to  precipitate  the 
gum.  If  the  substance  is  a colloid,  it  is  precipitated  with  the  gum 
and  colors  it.  Before  applying  this  test  it  is  necessary  to  assure 
one’s  self  that  alcohol  will  not  produce  a precipitate  in  the 
original  solution.  In  this  case  if  the  alcohol  did  not  exceed  50 
percent,  of  the  total  liquid,  nothing  was  precipitated,  so  care  was 
exercised  not  to  add  more  than  this  amount.  The  gum  when 
precipitated  was  colored  black,  and  on  standing  a few  hours  the 
supernatant  liquid  became  clear.  The  alcohol  was  then  poured 
off,  and  the  residue  treated  with  water,  which  dissolved  it  up 
again,  the  solution  being  colored  black  as  before.  This  is  the 
typical  behavior  of  a colloidal  substance.  Tyndall’s  experiment 
applied  by  Picton^  to  solutions  of  antimony  and  arsenic  sulphides 
was  tried  with  this  solution.  The  method  employed  was  essen- 
tially this  : The  liquid  in  question  was  placed  in  a small  bottle 
and  the  bottle  so  supported  that  the  rays  from  an  arc  light  could 
be  focused  directly  on  its  bottom.  The  light,  which  diffused 
from  the  side  of  the  bottle,  was  examined  through  a nicol.  A 
colloidal  solution  of  arsenic  sulphide  was  first  examined  to  see 
that  the  apparatus  gave  proper  results.  The  light  was  polarized, 
which  is  in  harmony  with  the  results  of  Picton  and  the  later  ones 
of  Stoekl  and  Vanino.^  The  solution  of  nickel  sulphide  was 
then  examined.  On  account  of  its  dark  color,  it  was  necessary 
to  use  a very  dilute  solution  so  that  the  light  would  not  be  com- 

1 Ber.  d.  chem.  Ges.,  20,  983  (1887). 

2 A Chem.  Soc.,  61,  143  (1892). 

Ztschr.  phys.  Chem.,%o,  iii  (1899). 


520 


O.  F.  TOWER. 


pletely  absorbed.  With  such  a dilute  solution  no  difficulty  was 
experienced  in  getting  it  into  the  bottle,  before  the  separation  of 
any  sulphur  due  to  oxidation  could  take  place.  The  light  diffused 
from  this  solution  was  completely  polarized.  Some  distilled 
water  from  which  the  solutions  were  made,  was  then  placed  in 
the  bottle  and  examined  in  the  same  manner.  No  polarization 
could  be  detected.  I am  greatly  indebted  to  Mr.  H.  W.  Wood- 
ward of  the  physical  department  of  this  university  for  very- 
efficient  aid  in  these  optical  investigations.  Independent 
observations  by  him  in  each  one  of  the  foregoing  cases  confirmed 
mine  in  every  particular.  All  these  tests,  therefore,  seem  to 
show  that  nickel  sulphide  exists  in  an  alkaline  tartrate  solution 
in  the  colloidal  state  ; that  is,  as  solid  particles  in  an  extremely 
finely  divided  condition,  too  small  to  be  seen  with  any  micro- 
scope but  not  so  small  but  that  they  reflect  light. 

cobaet  sulphide. 

Cobalt  sulphide  was  produced  in  a manner  exactly  similar  to 
that  used  in  preparing  soluble  nickel  sulphide.  A solution  of 
cobalt  tartrate  was  made  alkaline  with  a quantity  of  sodium 
hydroxide  sufficient  to  hold  nickel  sulphide  in  solution,  and 
then  hydrogen  sulphide  run  in  to  saturation.  The  cobalt  tartrate 
was  prepared  from  chemically  pure  cobalt  chloride  of  Kahlbaum’s^ 
marked  ''nickel  freiV  After  the  hydrogen  sulphide  had  been 
run  into  the  alkaline  solution,  it  was  filtered.  According  to 
Villiers,  the  cobalt  sulphide  is  entirely  precipitated,  but  in 
nearly  every  case  the  filtrate  was  colored  brown.  Villiers  points 
out  that  by  this  method  one  can  detect  the  presence  of  nickel  in 
most  preparations  of  cobalt  supposedly  free  from  nickel.  It  was, 
therefore,  expected  that  the  brown  coloration  just  referred  to 
was  due  to  the  presence  of  a trace  of  nickel.  This  brown  fil- 
trate was  refiltered  two  or  three  times,  then  shaken  with  sodium 
chloride,  and  allowed  to  stand  over  night.  In  the  morning  a 
slight  black  sediment  had  deposited  and  the  supernatant  liquid 
was  perfectly  clear.  The  deposit  was  filtered  off,  washed  and 
dissolved  in  aqua  regia.  After  removing  the  excess  »of  acid^ 
acetic  acid  and  potassium  nitrite  were,  added,  and  the  mixture 
left  twenty-four  hours,  at  the  end  of  which  time  a well-defined 


SULPHIDES  OF  NICKEL  AND  COBALT. 


521 


yellow  precipitate  had  appeared.  This  shows  conclusively  that 
the  brown  color  of  the  alkaline  solution  was  due  to  cobalt  sul- 
phide in  solution  and  not  nickel,  for  not  a trace  of  nickel  could 
be  found  in  the  filtrate  from  the  nitrite  precipitate.  This 
experiment  was  repeated  several  times  and  nearly  every  time 
some  cobalt  sulphide  went  into  solution.  The  presence  of  a 
moderate  quantity  of  a neutral  salt,  will,  however,  insure  com- 
plete precipitation  of  the  cobalt  sulphide. 

CONCLUSION. 

The  difficulties  attendant  on  the  separation  of  nickel  and 
cobalt  by  the  method  of  Villiers  are  the  following  : The  oxida- 
tion of  the  solution  to  which  the  hydrogen  sulphide  has  been 
added  results  in  the  separation  of  so  much  sulphur,  that  if 
nickel  is  present  and  cobalt  is  not,  the  black  solution  will  color 
the  sulphur,  making  it  very  difficult  to  distinguish  from  precipi- 
tated cobalt  sulphide.  Furthermore,  according  to  the  process 
used  in  this  separation,  sodium  chloride  is  present  in  the  solution, 
and  although  this  aids  in  the  complete  precipitation  of  cobalt 
sulphide,  it  may  cause  some  nickel  sulphide  to  be  precipitated, 
which  in  the  absence  of  cobalt  sulphide  might  readily  be 
mistaken  for  it. 

The  nickel  sulphide  in  solution  in  an  alkaline  tartrate  medium 
is  in  the  colloidal  state. 

Of  the  different  tartrates  of  nickel  and  cobalt  investigated,  the 
most  interesting  are  the  solutions  of  the  tartrates  of  these  metals 
prepared  from  their  hydroxides  and  cold  tartaric  acid.  The 
results  of  determinations  of  the  lowering  of  the  freezing-point 
and  of  the  electrical  conductivity  are  very  exceptional  for  salts 
of  this  sort,  and  seem  to  point  to  the  existence  of  a polymerized 
molecular  structure. 

The  evidence  seems  to  be  pretty  conclusive,  that  the  cause  of 
the  non-precipitation  of  the  hydroxides  of  nickel  and  cobalt 
from  solutions  of  their  tartrates,  or  at  least  that  any  precipitate 
that  may  form  dissolves  in  excess  of  the  reagent,  is  due  to  the 
formation  of  a compound  in  which  these  metals  replace  the 
hydrogen  atoms  of  the  alcoholic  hydroxyl  groups  of  tartaric  acid. 

Western  Reserve  University, 

Cleveland,  Ohio,  June,  1900. 


r ■ .r" 


